1. Field of the Invention
The invention concerns a method for establishing and connecting and disconnecting positioned anchorage points in different sea floor formations, together with equipment for the same, arising from the need which exists when anchoring floating units, independently of water depth.
When an object floating on the surface of the sea requires to be kept in position for various reasons, an anchor is employed. This consists of a heavy body, which is lowered on to the sea floor connected to a cable from the floating object. By means of its shape the anchor offers the possibility of becoming fixed to or exerting friction on the sea floor. For example, a ship which loses engine power at sea will make use of a traditional and simple anchor of this kind. In the same way this method of anchoring is employed by ships which are lying in the roadstead, waiting to put in at a quay, etc. The positioning requirements for ships in such circumstances are minimal, and the ship will normally be able to rotate freely 360.degree. round the mooring point, according to the state of the current and wind direction.
Floating objects, such as drilling platforms, production ships and the like, associated, e.g., with the oil and gas industry, have completely different and more stringent requirements for their positioning with consequent requirements for anchoring. This is due to the submerged pipe installations which extend approximately linearly from the drilling floor vertically through the water and on down deep to the oil and gas-bearing formations in the earth's crust.
Present day technology masters positioning of this kind down to a depth of approximately 700 meters, by the use of cable anchoring down and out from the platform, the number normally varying from eight to sixteen catenaries with attached plate anchors, or fluke anchors, at a cost of from NOK 1/2 mill. to 2 mill. These catenaries, e.g., are generally approximately four to six times as long as the distance to the bottom, and are deployed radially with the platform as the central point. In the outer end part of each chain there is attached a fluke anchor, which is designed to dig into the sea bed for securing co-operation with the sea floor when it is pulled over it towards the platform by anchor-taying vessels and/or the floating unit's own established tractive power.
Varying conditions on the sea floor and poor inspection capability reduce the certainty of secure and permanent anchoring in times of severe stress, with the result that unnecessarily stringent requirements are usually placed on the number of anchor points. Due to their high price, amongst other things, attempts must always be made to raise these fluke anchors for reuse.
Slack catenary mooring permits the platform, when exposed to wind and current forces, to drift in the horizontal plane in any direction from the central position to an extent corresponding to up to 5.degree. from the vertical plane.
Another method of attachment to the sea floor is a suction anchor. This is a metallic, bell-shaped anchor body with the opening facing down towards the sea floor. By means of a vehicle remotely operated from the surface of the sea, a ROV (Remotely Operated Vehicle), the water is pumped out of the body's internal volume, in order that the hydrostatic differential pressure at such depths should cause the body to be pulled/pressed down into and secured to the bottom. By this means a greater degree of controllable and inspectable attachment is achieved, thus permitting a substantially tauter mooring, with a shorter catenary.
Another remotely operated method of attachment at great depths is by ramming down hollow tubes by means of hydraulic hammer power, which tubes are thereby anchored in the bottom.
Securing by drilling in the bottom permits cylindrical hollow pipe anchors to be lowered, where cement is filled in cavities around and inside the cylinder. A catenary can then be attached to both the anchors' upper and top part projecting up from the bottom, or it is made fast to the anchors' central part projecting down into the bottom layer, in order thereby to exploit the resistance forces which arise when a body is pulled towards and through a surrounding mass.
Tension leg mooring is also employed, where anchors in the bottom with vertical catenaries attached to the stays counteract the platform's buoyancy by pulling it down in the water to an extent which has a stabilising effect.
Slack lines will occupy large areas in the sea and on the sea floor around a platform. It is undesirable for such lines to cross a flow line and/or another installation. Cables of metallic chain loops are heavy, also because each of these cables normally represents four to six times the sea depth. For example, a platform at 300 meters deep employing 10 catenaries of 1800 meters each will altogether have deployed 18000 running meters of chain. When the chain weighs 160 kg/running metre, the total weight is 2,900 tons. If a theoretical anchoring with the same means were performed at 3000 meters depth, the catenary weight would amount to 29,000 tons.
At such depths other catenaries have to be employed. Steel cable, e.g., weighs approximately a third of the weight of chain, and yet 3000 m of the dimension concerned weighs approximately 50 tons, forming an enormous coil. Composite cable systems will also be bulky, but such cables submerged in water are almost weightless.
A typical catenary can therefore be assembled by using large size steel cable or chain in the lower end part with a plate anchor to weight it down; from the floating unit steel cable or chain. The length between lower steel cable/chain and upper steel cable/chain is composed of composite fibre rope, the splicing being performed by means of special connecting units.
The method of the invention for establishing and connecting and disconnecting positioned anchorage points in different sea floor formations is primarily developed for operations at great depths with high hydrostatic pressure, which makes it difficult if not impossible, also from the cost point of view, to employ the present day known technology developed for moderate depths, for transferring, amongst other things, prevailing forces, catenary weights and dimensions, requirements for positioning, inspection, etc.
Known technical equipment which is employed in such subsea operations is a power-generating ROV (Remotely Operated Vehicle), which, at great depths with the necessary capacity with a hydraulic pump, produces the torque, tractive power and high liquid pressure for jetting and injecting effects.
A ROV is arranged to secured itself to the installation frame.
This is necessary to enable the ROV during turning work operations, such as screwing down wide-threaded cylindrical hollow threaded anchors or drilling in the seabed, to counteract the torque or recoil forces from high-pressure jetting and injecting to which it is exposed. The establishment of such power-generation on the installation frame is due to the fact that working at great depths complicates the operation supplying power from the surface.
From the patent literature the following publications are known:
NO 803927 describes a submersible percussion hammer which is surface-operated from a platform, which is supported by a truncated pyramidal frame which projects upwards from sea floor level.
NO 952476 describes a method for penetrating hollow cylindrical anchors in the sea floor, where the anchors with connecting means are coupled to a pillar of anchors stacked on top of one another, where the pillar's specific weight helps to ram one anchor after another down into the bottom, where these anchors are interconnected by lengths of chain which determine the distance between the anchors' chain-forming positions.
FR 2.444.755 describes a hollow helically flanked injector for anchoring and reinforcement of loose masses, in that after being screwed down into loose soil it permits a material which sets, e.g. liquid concrete, to be injected. The device is obviously surface-operated, and in itself does not represent an anchoring function.
SE 350.556 describes a percussion jetting device which is attached around the lower end part of a pile, which during surface-operated ramming into the ground with high-pressure water through obliquely downwardly jetting nozzles, achieves an easier/faster penetration in loose earth masses and the like.
PCT/WO 95/20075 describes a bell-shaped suction anchor coupled to and connected with one or more containers, where an underpressure has been created by pumping out water at a great depth. By repeated sudden opening and closing to the container's underpressure through the connection to the suction anchor's interior cavity, the shock-like pressure changes are transferred to the suction anchor, which penetrates the sea floor due to the hydrostatic differential pressure. The device which contributes to lowering the suction anchor here is the attached underpressure container(s). A standard suction anchor consists only of the bell-shaped body, which with its opening facing down on to the sea floor is first pumped empty of water, whereupon a high hydrostatic differential pressure builds up at great depths. By suddenly opening to this pressure, the suction anchor will be brought down into the bottom masses.
GB 2.148.968 describes a hollow cylindrical retrievable anchor, with outwardly and downwardly foldable curved arms suspended attached at 90.degree. to the anchor body. The anchor's function is to create concrete foundations under sea floor level, and then to be pulled up. The arms are folded into the lower part of a cylindrical anchor body, also by the pressure from the environment when being rammed down into the sea floor, but are folded out by the anchor being pulled slightly up and back, the arms' outer parts being turned inwards, thereby taking hold of the surrounding masses and on account of the resistance therefrom being forced into an oscillating movement from an enclosed position to a 90.degree. extended position on the anchor body. The ramming down operation is repeated by extending the anchor body by joining on new hollow units, and the longitudinal cavity is jetted with water. When the lower position has been reached, liquid concrete is added through the cavity. Retrieval for repeated use of the anchor is performed by lowering it further into the sea floor while jetting with water, with the result that the surrounding masses press the arms in towards the anchor body. The device may be described as an anchoring medium, and is presumed to be surface-operated, for reinforcement of the ground's supporting capacity, also because the force employed for the ramming down operation is not described.